uregex: allow \-escape before any non-letter
uregex: allow \-escape before any non-letter

Escape strictness makes hard to safely escape untrusted
strings - by putting '\' before any non-letter.

This applies only to extended regex syntax.  No such
clear rule for basic regexes.

mgjv/libusual,mgjv/libusual,ifduyue/libusual,markokr/libusual,rabbitt/libusual,rabbitt/libusual,rabbitt/libusual,rabbitt/libusual,ifduyue/libusual,mgjv/libusual,ifduyue/libusual,markokr/libusual,ifduyue/libusual,mgjv/libusual,markokr/libusual,markokr/libusual